A high-power RF amplification system for an antenna transmitter using optical components which permit the amplifying components that consume power and generate heat to be remotely located away from the RF load, which is at the antenna element. The amplification is performed by a set of optical fiber amplifiers arranged in parallel, with the amplified RF signal produced by electrically combining the outputs of a corresponding set of photodetectors.
For moderately narrow-band signals at high frequencies, an optical means, dual line lasers, can be added to achieve frequency conversion using the same set of photodetectors. Thus, a signal at baseband or intermediate frequency can be supplied to the amplification system, which produces a signal translated up to the band required for transmission from an antenna.
In addition, optical wavelength multiplexing can be used to combine or select the multiple optical-fiber amplifier channels. This permits the use of a single optical intensity modulator to transduce the RF input signal for the multiple optical fiber amplifiers, resulting in higher net RF gain from the amplification system.
Many RF antenna systems in the future will include electronically scanned phased arrays. Such antennas contain a large number (hundreds or thousands) of emitting or driven elements. Each of these driven elements requires a RF power amplifier of moderate power appropriate for the signal to be transmitted. For example, a 100-watt antenna composing of 500 driven elements requires an output of 200 mW per element. Typically, the power amplifiers are located at or near the antenna elements. This co-location of the amplifier at the antenna element is especially necessary for high-frequency antennas, which operate at RF frequencies from 30 to 90 Ghz, for example. Although amplifiers with large bandwidth and excellent power-added efficiency (PAE) are available for lower frequencies,  less than 20 GHz, high PAE and large bandwidth is difficult to achieve at the higher frequencies. For an exemplary 100-watt antenna, an additional 200 or more watts of excess power must be dissipated at the antenna aperture even for amplifiers with a PAE of  greater than 20%. This additional heat removal task further increases the bulk of the phased array antenna and also limits the flexibility of its design. The present invention provides a way to locate the amplifier, and its associated excess heat generation, remotely, away from antenna elements. The remote locating is accomplished by means of optical fibers, which have high immunity to electromagnetic interference (EMI) and which do not produce EMI.
A prior approach to realizing a high-frequency RF antenna transmitter is illustrated in FIG. 1. This approach is especially applicable to frequencies of 30 GHz and higher since it becomes increasingly difficult to obtain low-loss RF cables at such frequencies. According to this prior art approach, a low-band signal and one or more local-oscillator (LO) signals (two LO signals are shown in FIG. 1) are generated at units that can be remotely located from the driven elements of the antenna (not shown). Generally, several frequency conversion stages are used if a very high frequency signal is to be transmitted by the antenna. This multi-stage frequency conversion approach permits the frequencies of the LO signals to be lower. The frequency converters, such as a mixer (MXR), RF filters (BPFs) and RF electronic power amplifiers (EPAs) are located at the phased-array antenna elements.
In order to achieve the high output powers needed for a RF antenna transmitter, multiple electronic power amplifiers (EPA), connected in parallel, are typically used. One way to combine multiple RF amplifiers that are located at some, but only a moderate, distance from the load is to configure the amplifiers themselves as a RF antenna and then radiate the RF signal as a free-space electromagnetic wave to the load. See, for example, 2000 IEEE MTT-S Digest, paper WE2D-3, pp. 805-808. An output power of 5 watts was achieved at 37 GHz using an array of 512 transistor amplifiers. This use of free-space radiation between the amplifier and the load limits flexibility in the placement of the amplifier and load. Also, effects such as scattering of the radiated RF energy could be the cause of undesirable EMI.
Another way is to use a RF waveguide to combine the outputs of multiple transistor amplifiers. The combiner is fairly small, only a couple inches in length. A RF waveguide could be used to remote the load, but the loss of such waveguides is quite high, especially at the higher frequencies, and such waveguides can be quite bulky. Delivery of 2.2 watts power at W-band with 9.9% PAE using 8 MMIC-amplifier channels was reported in 2000 IEEE MTT-S Digest, paper WE4A-4, pp. 955-958. As expected, better performance was achieved at the lower frequencies. For example, 30 watts power at 31 GHz with a PAE of 20%, using a 3-stage configuration with 8 parallel output amplifiers was reported in 2000 IEEE MTT-S Digest, paper TU1F-42, pp. 561-563.
Given the smaller wavelength associated with these high frequencies and the desires to make the antenna lighter and to have a reduced radar cross section, there is a need to move (or remote) the power consuming and heat generating components away from the phased-array antenna elements. The present invention provides a way to locate remotely the RF power amplifiers and to eliminate the frequency converting mixers, which are the major power consumption and heat generation components at the antenna.
The approach of the present invention makes use of optical fiber amplifiers rather than electronic (transistor) amplifiers commonly used in the prior art. Also, the amplified signals are delivered to the load by means of optical fiber, rather than through free-space EM-wave propagation or by means of RF waveguides or RF cables. The heat generating portions of the amplifiers can be, and are preferably, located at a distance, perhaps many meters to hundreds of meters, away from the antenna and its driven elements. Multiple photodetecting devices are preferably used as transducers that convert the multiple amplified RF-modulated lightwave signals into multiple RF signals. The electrical outputs of the photodetecting devices are combined preferably at or near the driven elements. Electronic frequency converters (typically mixers) used in the prior art at the antenna are no longer needed since their function is performed by photodetecting devices.
The present invention achieves a high RF power output by adapting several prior approaches for combining multiple photodetectors. These prior approaches are described in more detail below and the distinctions between these prior approaches and the approaches of this invention are discussed below. The key distinguishing feature of the present invention, nevertheless, is the remotely located optical-fiber amplifiers and not the combination of photodetectors. The phrase xe2x80x9cremotely locatedxe2x80x9d is used herein to indicate that the heat generating sources associated with the optical-fiber amplification components can be located sufficiently distant from the antenna itself so that the heat generated by the heat generating sources does not impact antenna design considerations in any substantial way.